242 Mr. Shelford Bidwell on the 



§ 9. The Effect of Temperature upon the Resistance of 

 Crystalline Selenium. 



Text-books and dictionaries of chemistry appear to be about 

 equally divided upon the question whether the resistance of 

 crystalline Se is increased or diminished by rise of tempera- 

 ture. There are high authorities for both opinions. In a 

 paper published in 1883* I gave an account of some experi- 

 ments which indicated that the resistance of Se, " or rather 

 perhaps of selenium cells/' as I was careful to add, became a 

 maximum at a certain temperature, which varied a little with 

 different specimens but was generally a few degrees higher 

 than the average temperature of the air. Eight different 

 cells were tested, showing well-defined maxima at tempera- 

 tures of 13°, 14°, 22°, 23°, 23°, 24°, 25°, and 30° respectively. 



On further investigating the subject I have traced this 

 curious effect to the action of the fused-in electrodes. So far 

 as I know from a great number of experiments, a piece of 

 crystalline Se into which wires have been fused always 

 acquires a maximum resistance at a certain temperature. On 

 the other hand, the resistance of a piece having external 

 electrodes pressed into contact with it is always diminished 

 by a rise of temperature. 



Exp. 14. — A rectangular plate of Se was cast in one of 

 the white-glazed earthenware u pans " in which moist water- 

 colours are sold. Two platinum wires w r ere made red-hot, 

 and their ends embedded along the shorter sides of the rect- 

 angular plate a little below the surface. The plate was then 

 crystallized and annealed and its surfaces smoothed with 

 glass-paper. 



The plate was suspended inside an air-bath, the bulb of a 

 thermometer being in contact with it, and the resistance 

 betw r een the platinum-wire electrodes was taken at tempera- 

 tures ranging from —2° to 12°. The results, which are 

 given in the second column of Table II., indicate that the 

 resistance was greatest when the temperature was 6°. 



The same plate was then placed between tw r o pads of india- 

 rubber covered with tinfoil which served as electrodes, and 

 were pressed into good contact by a weight of 500 grammes. 

 The arrangement was placed in the air-bath, and the resist- 

 ance between the opposite faces of the plate observed at 

 various temperatures. As is shown in the third column of 

 Table II., the resistance diminished steadily as the tempera- 

 ture rose. 



* Phil. Mag. Jan. 1883, p. 31. 



